Semiconductor devices, such as discrete diodes and discrete insulated gate bipolar transistors, are typically formed in a semiconductor body including a lightly doped base region formed on a heavily doped substrate as the backside cathode/collector. The device region, such as the pn junction of the semiconductor device, is formed on the top or front side of the semiconductor body. In order to realize soft switching behavior in these semiconductor devices, especially when the thickness of the base region has to be kept thin, the semiconductor body often incorporates a field stop zone away from the device region and close to the backside substrate. The field stop zone is a region having the same doping type as the base region but with increased doping level as compared to the base region. The field stop zone has the effect of preventing the space charge region of the pn junction from propagating too far into the lightly doped base region. In practice, the field stop zone prevents the space charge region of the pn junction from reaching the backside cathode/collector. In this manner, the base region can be formed using the desired low doping levels and with the desired thickness while achieving soft switching for the semiconductor device thus formed.
Conventional methods for forming the field stop zone typically involve using high energy backside dopant implantation. The wafer is put through the front-side processing to form the device region and then the wafer is subjected to backside grinding to the desired thickness. Then, to form the field stop zone, one or more backside implantation is performed to introduce dopants into an area of the base region that is distant from the device region. For example, conventional methods typically use proton implantation or multiple helium or hydrogen implantations from the wafer backside to form the field stop zone. Then, a thermal anneal is carried out to activate the hydrogen-related donors. FIG. 1 duplicates FIG. 1 of U.S. Pat. No. 7,538,412 and illustrates an IGBT formed including a field stop zone 26 formed by high energy backside implantation. FIG. 2 duplicates FIG. 2a of the U.S. Pat. No. 7,538,412 and illustrates an example doping profile of the field stop region as a result of the multiple backside implantations.
The conventional methods for forming the field stop zone in a semiconductor body have many shortcomings. First, when high energy backside implantation is used, it is difficult to form deeply extending field stop zone, which requires extremely high implantation energy which is either not feasible, limited by implant equipment, or not manufacturable or associated with unaffordable cost.
Second, when high energy backside implantation is used, the ability to form the desired field stop zone doping profile becomes limited. In some cases, a large number of implantations is needed to form the desired doping profile. Large number of implantations is not desirable and can be costly.
Lastly, since the backside implantation and anneal are carried out after wafer front-side processing is done, the anneal temperature for the backside implant cannot be too high. For example, the anneal temperature for the backside implant cannot exceed 500° C. or 550° C. because of the metallization layer formed on the front side. The available anneal temperature range limits the implantation dose and energy that can be used for the backside implant because higher implant dose or higher implant energy requires longer thermal process to anneal the implant damage or defects.